Several prior art sound reproducing systems have included the loud speaker in a feedback path for reducing loud speaker distortion, and for allowing the use of smaller loud speakers and loud speaker enclosures. Such prior art systems, especially those which include means for magnetically sensing the axial motion of the associated loud speaker cone, have considered neither the detrimental effects due to electrical interference from the main electromagnetic loud speaker structure, nor the proper frequency shaping of the motional feedback signal to cause the loud speaker to respond linearly to the input source signal. Such prior art sound reproducing systems are cited in U.S. Pat. No. 3,798,374 entitled, "Sound Reproducing System Utilizing Motional Feedback", issued on Mar. 19, 1974 to Stanley Thayer Meyers, Applicant herein.
Further, U.S. Pat. No. 3,821,473 entitled, "Sound Reproduction With Driven And Undriven Speakers And Motional Feedback", issued on June 28, 1974 to Mullins discloses amplifier 4, device 12, and speakers 14 and 16. In such system, there is included an undriven speaker and each of the speakers mounted in the enclosure have different resonant frequencies and motional feedback devices attached thereto. The outputs of the motional feedback devices are combined to provide a negative feedback signal to the amplifier. The system also includes motional sensor 18. Such sound reproduction system relates to combined motional feedback control of driving and driven speakers in a single enclosure. There does not appear to be a description of any particular type of motional feedback sensing means, although acceleration sensing is mentioned.
U.S. Pat. No. 3,878,748 entitled, "Oral Cavity Controlled Electronic Musical Instrument", issued on Apr. 22, 1975 to Spence discloses sensor coil 58. FIG. 9 of such patent refers to a method of divesting a separate sensing coil of interference from the voice coil. Such arrangement appears to be a ramification of bridge type feedback control.
U.S. Pat. No. 4,025,722 entitled, "Method And Apparatus For Recording", issued on May 24, 1977 to Karron discloses speaker 20 including voice coil 18 and auxiliary winding 30. The output of auxiliary winding 30 is coupled to primary winding 32 of transformer 34, but does not appear to be fed back to amplifiers 16 thereof.
The North American Philips Corporation distributes a sound reproducing system including a signal source, an electronic crossover, a comparator, a low frequency amplifier, a woofer, a piezoelectric sensor, a high frequency amplifier, a second crossover, a mid-range speaker, and a tweeter speaker. In such sound reproducing system, acceleration feedback is utilized but only in the so-called woofer speaker.
However, none of the aforementioned prior art sound reproducing systems includes the particular cone motion sensing structure to produce a motional feedback signal according to the present invention.
Objects of the present invention are therefore to:
Utilize motional feedback in a sound reproducing system for reducing loud speaker distortion, for providing a uniform sound energy output, and for effecting linear loud speaker response to the input source signal;
Utilize motional feedback in a sound reproducing system wherein relatively small loud speakers and loud speaker enclosures are required; and
Utilize the stray magnetic field of the loud speaker main electromagnetic structure to provide a motional feedback signal readout of the loud speaker cone.